I think I see water in both, maybe not.
different colors for sure.
Ok just for fun and giggles.
The mallory is a 10 micron.,
I would "think" the mercury is the same, and even thinking further that all marine separators would be industry standard the same.
Ethanol from my experience and findings almost draws moisture to it, or it has moisture and it separates with time.
Anytime, I work on the fuel system for a "fuel problem,"
I most times encounter water, in the bowl or filter, thats atvs, mowers, tractors, tillers, generators, especially small equipment.
I have for the last 3-5 years swore of of putting Reg. pump gas in equipment of mine.
I keep Eth free on hand and I tend to add 1 oz per gallon of cheap 2 stroke oil to everything. I do that for my diesel too,
just a little lube for needle & seat and combustion, it in theory runs everything technically said leaner, but I'll trade the slick'em for it.(turns my gas a pale carolina blue tint)Got a tad off topic, but its distantly related.
but back to the filter you can separate water from gas with a napkin single layer, a quad folded napkin opened up laid in a funnel will separate water from gas or diesel.
Thats one way I finish separating fuels. to not just throw gas away.
My point is in all this, that its not hard to separate water from gas, even an old school clear glass screen in-line or also a sediment bowl again only a screen not "filter material" will screen out water unless you get massive amounts of water.
I say theres nothing magical about the 2 different filters.
Sorry for the long spill but I have dealt with this a lot, not the twin thing but water in fuel.
I too would tend to think the pick up is slightly different and a little will do ya, unless you were really sloshing as little as a 1/4"-1" would scavenge water before it got to the second pick up.
you just had the perfect storm situation.
And this could be a design thing to let 1 engine stall first and the second possibly limp on, just by luck.
It might be worth noting for thought.
or maybe I rambled for nothing in the wrong direction.
Again sorry if I rambled, not a simple explained topic.